Specific techniques to avoid interference in wireless communication systems are known in the art. For example, U.S. Pat. No. 5,048,015 discloses the use of an identification code which is transmitted within signals to identify the presence and source of identification. U.S. Pat. No. 5,148,548 prioritizes channels for use based on their likelihood that they would cause interference if used. U.S. Pat. No. 5,258,997 uses modulation techniques in a spread spectrum system to avoid interfering with narrowband systems. U.S. Pat. No. 5,255,514 avoids selecting channels which may interfere with calls in neighboring cells or which may suffer substantial interference from a neighboring base-station.
The current Code Division Multiple Access (CDMA) wireless communication system envisioned for, inter alia, cellular applications in the United States is defined by TIA/EIA/IS-95, Mobile Station-Base Station Compatibility Standard for Dual Mode Wideband Spread Spectrum Cellular System, July 1993. To implement CDMA, many system operators are clearing out a block of frequencies and placing CDMA carriers within the block. These CDMA carriers are typically placed next to the narrowband channels already installed in a system, as shown in FIG. 1.
Referring to FIG. 1, the typical channelization plan where two narrowband carriers 100, 101 are adjacent to two wideband (CDMA) carriers 103, 104 is shown. As is clear from FIG. 1, the CDMA carriers 103, 104 are much wider than the narrowband carriers 100, 101. Additionally, the guard band 105 between the CDMA carrier 103 and the narrowband carrier 101 is insufficient to protect a CDMA mobile station communicating on the CDMA carrier 103 from interference due to the narrowband carrier 101 when the CDMA mobile station is close to a base-station (not shown) which transmits the narrowband carrier 101. This is due to the practical limitation in the design of a wideband receiver implemented in a CDMA mobile station and its inability to sufficiently reject the adjacent narrowband carrier. Because of this, degradation of the communication (the call) occurs when the CDMA mobile station is within a certain distance of the interfering narrowband base-station. In addition to the effect of single adjacent narrow band carriers, combinations of carriers can produce degradation due to other mechanisms including intermodulation.
Thus, a need exists for an improved wireless communication system which overcomes the shortcomings mentioned above.